Manufacture of Paper
What is Paper?
Paper is an aqueous deposit of any vegetable fibre in sheet form. If you tear a piece of paper
you will see a number of minute ‘whiskers’ sticking out from the line of the tear. These are the
fibres, they are very small in size (not much more than a millimetre in length) and there are
millions of them in any piece of paper.
Paper is made from cellulose, trees being the main source of cellulose
fibre (or woodpulp). Besides woodpulp, paper can be made from other
materials such as cotton, flax, esparto, straw, hemp, manilla and jute.
Some of the pulp properties depend upon the process used to separate
the fibres from the timber. The main processes are called mechanical
and chemical. (See Making Paper from Woodpulp).
If the pulp mill is close to the paper mill, i.e. it is part of an integrated mill,
the liquid pulp is pumped direct to the paper mill. Otherwise, after
straining, the pulp is dried and packed into bales. The trees used in
Spruce Wood Fibres
papermaking are mainly grown in Scandinavia, North America and
Canada. They are cone-bearing, for example, spruce, pine or fir
softwoods), and leaf-bearing trees such as birch, beech or eucalyptus (hardwoods).
Recovered paper is a very important source of fibre for the papermaking industry. It is called
secondary fibre to distinguish it from the fibre obtained directly from wood (sometimes called
primary fibre). Before Recovered paper can be turned into pulp it must be collected, sorted,
graded and baled. (See Making Paper from Waste). It is then transported to the mills where
further processes are carried out to remove unsuitable materials (contraries) e.g. paperclips,
staples and plastic. It is then broken down into pulp and sometimes printing ink is removed
from the slurry. (See De-Inking).
In the Paper Mill
The bales of dry pulp are broken up in a hydrapulper (a large
circular metal tank in which a mixture of pulp and water circulates).
The liquid pulp, looking a little like porridge, is then pumped to
machines known as refiners. These modify the fibres by passing
them between sets of fixed and rapidly moving metal bars. As the
fibres undergo this treatment they are chopped or bruised to
varying extents. This operation, sometimes referred to by its older
name of ‘beating’, is extremely important as it mainly controls the
type of paper being made.
The pulp is now known as stock. It is at this point that the size and colours are added. Size
consists of starch or resin whose particles settle on the fibres, producing a surface that does
not blur when inked. Dyes are added to colour the paper. Even if the paper is to be white,
some colouring is added, as without it the paper has a slightly yellowish look. Quite frequently
mineral matter such as china clay is added to the stock, to make the paper less transparent.
Such materials are known as fillers.
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The Papermaking Process
Recovered Paper
De-inking
Cleaning
Chemical Pulping
Timber
De-barker
Blending
Chipper
Mechanical Pulping
Refining
Screening & Cleaning
Drainage…..Pressing…..Drying…..Smoothing
Coating
Conversion
(cutting,
sheeting)
Distribution
End Use
Now the actual process of changing stock into paper begins. A typical paper machine is
approximately 100m long and 4m wide, and can cost over £200 million to build. The machine
has a ‘wet end’ and a ‘dry end’. At the wet end, the stock is diluted to 200 parts water and 1 part
fibre. The diluted pulp, looking a little like milk, is pumped through a horizontal slit, from where it
lands on a fast moving plastic mesh called the wire. The wire is made to vibrate and it has
suction equipment below it so that water drains rapidly, leaving the fibres as a mat on top of the
mesh. The wire can travel at speeds of up to 2000m/minute. By the time it reaches the end of
this section the fibres have become a sheet of paper, though very moist and having little strength.
It then passes to the press section. This consists of a number of sets of heavy rollers through
which the moist paper is conveyed, on thick felts. Even at this stage the paper ‘web’ is still weak
and moist.
It then passes to the dry end which consists of a large number of steam heated drying cylinders,
up to 100 or more on large paper machines. Part of the way down the bank of drying cylinders is
the size press. It is here that a solution of water and starch
can be added to the sheet in order to further improve
the surface. Beyond the drying cylinders comes the
calender, which consists of a stack of polished iron
Paper can travel up to speeds of
rollers mounted one on top of the other. Its function is to
60 miles per hour on a
polish or glaze the surface of the paper as it passes
paper machine.
between the rollers.
Still travelling at up to 2000m/minute, the paper now comes off the machine ready for ‘kindling’
into large reels, each of which may contain up to 20 tonnes of paper. These reels are later either
cut into sheets or slit into smaller reels according to the type of paper and the customer’s
requirements.
A separate process is the coating of the paper with china clay and then polishing it through the
calender. This makes art paper, the glossy kind used for pictures. There are 3 main kinds of
modern coating processes: blade coating, air knife coating and roll coating. Blade coating is
normally done off the paper machine, as is air knife coating. But roll coating is done on the paper
machine and can either be a complete coating or a first coat followed by ‘off-machine’ coating by
the blade or air knife process.
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